1. Delivery, Forwarding and
Chapter 22
Network layer
Delivery, Forwarding and
Routing
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

2. Network layer Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

3. Direct and indirect delivery
Dr. Gihan Naguib

4. Topics discussed in this section:
FORWARDING
Forwarding means to place the packet in its route to its destination. Forwarding requires a host or a router to have a routing table. When a host has a packet to send or when a router has received a packet to be forwarded, it looks at this table to find the route to the final destination.
Topics discussed in this section:
Forwarding Techniques Forwarding Process
Routing Table
Dr. Gihan Naguib

5. Route method versus next-hop method
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6. Host-specific versus network-specific method
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7. Host-specific routing
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8. Default method
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9. Simplified forwarding module in classless address
Note
In classless addressing, we need at least four columns in a routing table.
Dr. Gihan Naguib

10. Example 1:
Make a routing table for router R1, using the following configuration m3
(/0)
( )
Dr. Gihan Naguib

11. Example
Show the forwarding process if a packet arrives at R1 with the destination address
Routing table for R1:
(/0)
( )
Dr. Gihan Naguib

12. Solution: The router performs the following steps:
1. The first mask (/26) is applied to the destination address. The result is , which does not match the corresponding network address.
2. The second mask (/25) is applied to the destination address. The result is , which matches the corresponding network address. The next-hop address and the interface number m0 are passed to ARP for further processing .
Dr. Gihan Naguib

13. Example
Show the forwarding process if a packet arrives at R1 in with the destination address
Solution: The router performs the following steps:
1. The first mask (/26) is applied to the destination address. The result is , which does not match the corresponding network address.
2. The second mask (/25) is applied to the destination address. The result is , which does not match the corresponding network address (row 2).
3. The third mask (/24) is applied to the destination address. The result is , which matches the corresponding network address. The destination address of the packet and the interface number m3
Dr. Gihan Naguib

14. Example
Show the forwarding process if a packet arrives at R1 in Figure 22.6 with the destination address
Solution
This time all masks are applied, one by one, to the destination address, but no matching network address is found. When it reaches the end of the table, the module gives the next-hop address and interface number m2 to ARP. This is probably an outgoing package that needs to be sent, via the default router, to someplace else in the Internet.
Dr. Gihan Naguib

15. Routing
Packet go from source to destination via routers.
Routing means to place the packet in its route to its destination.
Routing requires a host or a router to have a routing table. When a host has a packet to send or when a router has received a packet to be forwarded, it looks at this table to find the route to the final destination
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking

16. Routing Table Static : Dynamic:
does not change automatically (manual entries)
Dynamic:
Updated automatically when there is change in the Internet.
For instance, they need to be updated when a router is down, and they need to be updated whenever a better route has been found
Routing protocols are needed to create the routing tables dynamically.
A routing protocol is a combination of rules and procedures that:
Lets routers in the internet inform one another of changes.
Allows routers to share whatever they know about the internet or their neighbourhood.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking

17. Unicast Routing
In unicast routing, there is only one source and only one destination.
When a router receives a packet, it forwards the packet through only one of its ports (the one belonging to the optimum path) as defined in routing table.
Discard the packet, if there is no route.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking

18. Autonomous systems
Autonomous System (AS): Group of networks and routers under the authority of a single administration.
Routers inside an autonomous system is referred to as interior routing (Intradomain).
Routing between autonomous systems is referred to as exterior routing (Interdomain).
Solid lines show the communication between routers that use interior routing protocols.
Broken lines show the communication between routers that use an exterior routing protocols.
Behrouz A. Forouzan “Data communication and Networking
Dr. Gihan Naguib

19. Interior and Exterior routing protocols
Each AS can choose one or more intradomain(interior) routing protocol to handle routing inside the AS such as RIP and OSPF
One interdomain (exterior) routing protocol is usually chosen to handle routing between ASs ; BGP
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

20. Metric of different protocols
Metric is the cost assigned for passing through a network.
The total metric of a particular router is equal to the sum of the metrics of networks that comprise the route.
A router chooses the route with smallest metric.
The metric assigned to each network depends on the type of protocol
RIP (Routing Information Protocol): “Shortest distance”
Cost of passing each network is same; it is one hop count.
If a packet passes through 10 networks to reach the destination, the total cost is 10 hop counts.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking

21. Metric of different protocols
OSPF (Open Shortest Path First):
Administrator can assign cost for passing a network based on type of service required such as :
throughput, delay,..etc.
OSPF allows each router to have more than one routing table based on required type of service
BGP (Border Gateway Protocol):
Criterion is the policy, which is set by the administrator.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking

22. Routing Algorithm classification
Routing algorithms can be:
distance vector algorithms:
router knows physically-connected neighbors, link costs to neighbors, iterative process of computation, exchange of partial information with neighbors.
The least cost between any two nodes is the route with minimum distance
Routing Information Protocol (RIP) is an implementation of this approach
2. link state algorithms:
all routers have complete topology, link cost information
Open Shortest Path First (OSPF) is an implementation of this approach.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

23. Distance Vector Routing
Each node( router) maintains a set of triples (table): Destination, Cost and Next Hop
Node knows the cost to each neighbor (the distance between itself and its immediate neighbors)
Directly connected neighbors exchange updates
periodically (on the order of several seconds -30s)
whenever table changes (called triggered update)
Each update is a list of pairs: Destination, Cost
Update local table if receive a “better” rout (smaller cost)
Dr. Gihan Naguib

24. Distance vector routing
Initialization of tables in distance vector routing:
infinite ∞ ( unreachable).
Think the node as the cities and the lines as the roads connecting them
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

25. Distance vector routing
Sharing: Updating in distance vector routing
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

26. Distance vector routing
The least cost route between any two nodes is the route with min distance.
Each node maintains a table which contains : Destination, Cost, Next hop
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

27. Distance vector routing
Each router shares its entire routing table with its neighbours.
Sharing:
periodically update :on the order of several seconds -30s-
Triggered update: The change can result from the following:
A node receives a table from a neighbor, resulting in changes in its own table after updating.
A node detects some failure in the neighboring links which results in a distance change to infinity ∞
Each update is a list of pairs: Destination, Cost (two column routing table)
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

28. Routing Information Protocol (RIP)
RIP is based on Distance vector routing:
RIP implement Distance vector routing with some considerations:
Destination in the routing table is a network (first column defines network address)
Metric(distance) is Hop count : is the number of networks that a packet encounters to reach its final destination
Infinity is defined as value of 16 .
Therefore, the Max limited of hops is 15
suitable for small networks (local area environments)
Router sends update message to neighbors every 30 sec.
If router does not receive update message from neighbor X within this limit, it assumes the link to X has failed and sets the corresponding minimum cost to 16 (infinity).
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

29. immediate neighbors periodically and when there is a change.
Note
In distance vector routing, each node shares its routing table with its
immediate neighbors periodically and when there is a change.
Dr. Gihan Naguib

30. Routing Information Protocol (RIP)
Routing table
Has one entry for each destination network of which the router is aware.
Each entry has destination network address, the shortest distance to reach the destination in hop count, and next router to which the packet should be delivered to reach its final destination.
Destination
Hop Count
Next Router
Other information 7 5 4 6
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

31. Example Internetwork Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

32. Initial routing tables in a small autonomous system
When a router is added to a network, it initializes a routing table for itself, using its configuration file.
The table consists only the directly attached networks and the hop counts, which are initialized to 1.
The next-hop field, which identifies the next router, is empty.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

33. Updating Routing Table for Router A
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

34. Final routing tables
Each routing table is updated upon receipt of RIP messages using the RIP updating algorithm.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

35. RIP Updating Algorithm
Receive: a response RIP message(significant portion of its routing table)
1. Add one hop to the hop count for each advertised destination.
2. Repeat the following steps for each advertised destination:
1. If (destination not in the routing table)
1. Add the advertised information to the table.
2. Else
1. If (next-hop field is the same)
1. Replace entry in the table with the advertised one.
1. If (advertised hop count smaller than one in the table)
1. Replace entry in the routing table.
3. Return.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

36. Example of updating a routing table
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

37. Example of a domain using RIP
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

38. Link State Routing
Each node in the domain has the entire topology of the domain.
Analogous to a city map.
Each node uses the same topology to create a routing table.
Dr. Gihan Naguib

39. Building Routing table
Creation link state packet (LSP)
LSP is a tabular representation of the topology of the internet
Every router in the same area has the same LSP
After a node has prepared an LSP , it must be disseminated to all other nodes . The process is called Flooding
Formation of shortest path tree for each node (router) using Dijkstra algorithm
Calculation of routing table.
Dr. Gihan Naguib

40. Building Routing table
Creation link state packet (LSP):
LSP can carry a large amount of information such as : the node identity, list of links, sequence number,…etc.
LSP are generated on two occasions:
When there is a change in topology of the domain. Triggering of LSP dissemination to inform any node in the domain to update its topology
On a periodic basis. The period in this case is much longer compared to distance vector routing . The timer set for periodically dissemination is in the range 1 or 2 h.
It done to ensure that the old LSP is removed
A longer period ensures that flooding doesn’t create much traffic on the network
Dr. Gihan Naguib

41. Building Routing table
Flooding
The creating node sends a copy of the LSP out of each interface
A node receives an LSP compares it with the copy it has. If the newly arrived is older than one it has , it discards
If it is newer the node do the following :
It discards the old LSP and keep the new one
It sends a copy of it out of each interface except the one from which the packet arrived.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

42. Dijkstra Algorithm
Calculates the shortest path between two points on a network, using a graph made up of nodes and edges.
Algorithm divides the nodes into two sets: tentative and permanent. It chooses nodes, makes them tentative, examines them, and if they pass the criteria, makes them permanent.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

43. Examples of formation of shortest path tree usingDijkstra algorithm
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

44. Examples of formation of shortest path tree usingDijkstra algorithm
Routing table for node A
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

45. Open Shortest Path First :OSPF
OSPF Based on Link state Routing
OSPF divides an autonomous system into areas.
Special routers called autonomous system boundary routers are responsible for dissipating information about other autonomous systems into the current system
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

46. Areas in an Autonomous System
Area is a collection of networks, hosts, and routers all contained within an autonomous system.
Routers inside an area flood the area with routing information.
Area border routers: Summarize the information about the area and send it to other routers.
Backbone area [Primary area]: All the areas inside an autonomous system must be connected to the backbone. Routers in this area are called as backbone routers. This area identification number is 0.
If, due to some problem, the connectivity between a backbone and an area is broken, a virtual link between routers must be created by the administration to allow continuity of the functions of the backbone as the primary area.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

47. OSPF
Each router should have the exact topology of the internet at every moment.
From this topology, a router can calculate the shortest path between itself and each network using Dijkstra algorithm
Types of Links
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

48. Types of Links 1. Point-to-point
Connects two routers without any other router or host in between.
Directly connected routers using serial line.
Only one neighbour.
No need to assign a network address to this type of link
The metrics are the same at the two ends
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

49. Types of Links 4. Virtual 2. Transient link 3. Stub
A network with several routers attached to it.
Each router has many neighbours.
3. Stub
A network that is connected to only one router. The data packets enter the network through this single router and leave the network through this same router.
4. Virtual
When the link between two routers is broken, the administration may create a virtual link between them, using a longer path that probably goes through several routers.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

50. Example of an internet & Graphical representation
Point to point : between A_D and B-E
Stub: N5, N4 and N2
Transient : N1 and N3
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

51. Shortest-path calculation using Dijkstra
Example 2:
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

52. Shortest-path calculation using Dijkstra
Example 2:
Shortest-path calculation using Dijkstra
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

53. Shortest-path calculation using Dijkstra
Example 2:
Shortest-path calculation using Dijkstra
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

54. Shortest-path calculation using Dijkstra
Example 2:
Shortest-path calculation using Dijkstra
Routing Table for Router A
92 permanent
66 permanent
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

55. Shortest-path calculation using Dijkstra
Example 3:
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

56. Shortest-path calculation
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

57. Shortest-path calculation
Network
Cost
Next Router N1 5
---- N2
  7 C N3 10 D N4 11 B N5 15
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

58. OSPF
Reaction to Failure
If a link fails,
Router sets link distance to infinity & floods the network
with an update packet
All routers immediately update their link database (LSP)&
recalculate their shortest paths
Recovery very quick
NOTE:
OSPF differs from RIP in that each router knows the exact topology of the network
OSPF reduces routing bandwidth usage
OSPF is faster than RIP.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

59. Multicasting and Broadcasting
In multicast communication, there is one source and a group of destinations.
The relationship is one-to-many.
In this type of communication, the source address is a unicast address, but the destination address is a group address, which defines one or more destinations.
Dr. Gihan Naguib
Behrouz A. Forouzan “Data communication and Networking”

60. In unicasting, the router forwards the received packet through only one of its interfaces
In multicasting, the router may forward the received packet through several of its interfaces.
Dr. Gihan Naguib

61. Broadcasting
In broadcast communication, the relationship between the source and the destination is one-to-all.
There is only one source, but all the other hosts are the destinations.
The Internet doesn’t explicitly support broadcating because of the huge amount of traffic it would create
Dr. Gihan Naguib

62. Reading from the text book
Chapter 22 from 4th edition:
Section 22.1
Section 22.2 until page 652 (example 22.3 only)
Section 22.3 until page 674 except Two-Node Loop Instability and Three-Node Loop Instability)
Section 22.4 until page680 (Broadcasting)
Dr. Gihan Naguib